Tracking issue for `time_checked_add` feature

[sgeisler]

This is a tracking issue for PR #55527 .

This feature adds a checked_add(&self, &Duration) -> Option<SystemTime> function to std::time::SystemTime and as a prerequisite also for all platform specific time structs. This also led to the refactoring of many add_duration(&self, &Duration) -> SystemTime functions to avoid redundancy (they now unwrap the result of checked_add_duration).

[Amanieu]

This should be added to Instant as well. We're seeing some need for this in parking_lot to handle wait timeouts.

[faern]

I pushed a PR (#56490) for adding the same functionality to Instant under the same feature gate name. I hope this is fine. They seem extremely related (and share a lot of the implementation) and the feature name fits both.

[faern]

This has now been extended so the feature covers both checked_add and checked_sub on both Instant and SystemTime. Together with the fact that Duration already have these, it's now possible to handle all time types without risking panics on nightly (whenever the next nightly comes out).